1. Field of the Invention
The present invention relates generally to lithography, and more particularly to lithography focus control apparatus and methods.
2. Related Art
A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. including part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction, also referred to as the “y-direction”) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
In manufacturing a device by lithography, focus control of the radiation beam onto the substrate is of paramount importance. One key source of error in focus control is the inherent unevenness of the substrate, for example the wafer. If the local topology of the substrate is mapped and known, this unevenness can be partially compensated for in the scanning direction by adjusting the patterning device in the vertical direction (“z-direction”) according to the mapped topology of the substrate. However, this does nothing to correct for focus errors in the non-scanning (“x-direction”). The patterning device can also be rotated about axes parallel and perpendicular to the scanning direction, but this only partially corrects for focus errors in the x and y directions. Because of these reasons, focus control in the x-direction is a major challenge. In other cases, even the inherent unevenness of the patterning device itself can introduce errors into focus control.